Tone unit



P 1949" o. D. GRANDSTAFF TONE UNIT Filed March 29, 1947 E: 1 v 09 czpmz INVENTOR. Ofho D. Grandsiaff 8E 50 @222 El 528 W5 W222 56: 50 m 8o ugg 1910a 93 pemgoossv 01 Afforneys Patented Sept. 20, 1949 UNITED sr 'r Es PATENT ori ice 7 TONE UNIT Otho 1).; ,Grandstafi; Oak Park,.Ill., assignor to 7 Automatic Electric Laboratories, Inc., Chicago, I 11 1., a corporation of Delaware Application March 29, 1947, Serial No. 738,194 a The present invention relates to telephone systems and, more particularly, to tone units incorporated therein.

A telephone system of the type disclosed in United States Patent No. 2,155,176, granted April 18, 1939, to John Wicks, comprises atwoway toll line terminated at the opposite ends thereof in repeater circuits provided with associated tone units of the character disclosed in United States Patent No. 2,159,081, granted May 23, 1939, to Otho D. Grandstaff. The tone unit of the present application constitutes an improvement in the tone unit of the Grandstaff patent mentioned and is adapted to be incorporated in the telephone system disclosed in the copending application of John Wicks and Otho D. Grandstaff, Serial 749,758, filed May 22, 1947; which last-mentioned telephone system consti tutes an improvement in the telephone system of the Wicks patent mentioned. I

In the telephone system .of the Wicks patent, an operator at an operator position in a first exchange may extend a connection over the toll line to switching apparatus in a'seco'ndjexchange, and then control the switching-"apparatus further to extend the connection. During these operations a first tone unit associatedwith a first repeater circuit, associated *withiafirst end of the toll line in the first exchange, cooperates with a second tone unit associated with a second repeater circuit, associated with a second end of the toll line in the second exchange. Each of the tone units is capable of transmitting and of receiving both umnodulated 1000 cycle signals and 50-60 cycle modulated 1000 cycle signals, which are utilized for the various dial and supervisory purposes. I

It has been found'that it is desirable for the operator at the operator position in the first eX- change to monitor the further extension of the connection by the switching apparatus in the second exchange; which direct monitoring arrangement in the telephonesystem of theWicks patent mentioned is not altogetherrsatisfa'ctory since it tends to interfere 'with'th'e reception'of supervisory signals by the first tonexunit-in the first exchange, that are transmitted over they toll line from the second toneunit-in the second exchange, during the further extension .of tlie'con-- nection.

Moreover, it has been found that the tone unit of the Grandstaff patent mentioned has a tend ventionto provide a tone-unit of the character described that is capableof renderingmonitoring supervision to an associated operator po ieclaims. (01. 179- 16) tion during the extension of a connection over an associated toll line, without in any way interfering with the concurrent-reception ofsupervisory signals thereby.

Another object of the invention is to provide a tone unit of the character described that is arranged to render to an associated operator position both audible and visual supervisory. signals during the extension of a connectionv over an associated-toll line.

Another object of the invention is to provide amplifying apparatus capable of receiving a1- ternating current signals over a considerable range of intensities and comprising an improved arrangement for preventing overloading thereof incident to thereception of such signals that exceed in intensity the upper limit of the range mentioned. 1

A further object of the invention is to provide amplifying apparatus comprising a tuning arrangement rendering the same selectively responsive to alternating current signals within a given frequency band and an improved arrangement for limiting the response thereof to strong signals within the band mentioned.

A further" object of the invention is to provide a tone unit of the type described embodying improved and simplified circuit arrangements for transmitting and for receiving both modulated and unmodulated alternating current signals.

Further features of the invention pertain to the particular arrangement of the circuit elements of the tone unit, whereby the above outlined and additional operating features thereof are attained.

The invention both as to its organization and method of operation, together with further objects-and advantages thereof, will best be understood by reference to the following specification taken in connection with the accompanying drawing in which the single figure illustrates diagrammatically a tone unitconnected and arranged in accordance with the present invention. Y 'Referring now to the drawing, the tone unit there illustrated embodies the features of the invention and essentially comprises a receiving transformer TRI, a sending and coupling transformer TR2, a monitoring'transformer TR3, a two-way amplifier Tl in theform of a vacuum tube of the well-known pentode 6C6 type, a differential detector T2 in the form of a vacuum tube ,of the well-known dual diode 6H6 type, an impulse amplifier T3 in the form of a vacuum tube of the well-known pentode 606 type, and a signal amplifier T4 in the form of a vacuum tube of the well-known pentode 43 type. Also, the tone unit I00 comprises a relay group including a modulation cutoff relay RIZO, areceiving switch relay RI-lfl, a pulse cut-in relay R150, an alarm relay R155, an impulse-in relay R and a signal-in relay RI'IIJ; as well as a plate impedance I23 associated with the tube TI a plate impedance I30 associated" wi th" the tube T3, and a plate impedance I39 associated with the tube T4. Further, the tone unit I comprises circuit ele: ments described in detail hereinafter and interconnecting the elements mentioned; as well as a group of conductors extending to the associated repeater circuit and including a sending -;conductor C I 0 I a sending condu'ct'or C I02, toring conductor CI 03, a monitoring conductor CI 0d, a receiving conductor CI05, a receiving conductor CI06, a receiving switch conductor CIEB'I, a modulationputofi conductor CI'OB, a signal-in conductor CI 09, a dial-in conductor CI m and a purse cut-in conductor CI I I; together with a '0-60cycle conductor C I' I 2' extending to an associated 50-60 cycle generator, not shown,,and' a 1000 cycle conductor CI l3 extending to a 1000 cycle generator, not shown. I

The tubes TI and T3 are each provided with a cathode heater having a .nominal rating 'of v6.3' volts, at which voltage 0.3 ampere is drawn; the tube T2 is provided with a two-section cathode heater having a nominal rating of 6.3 volts, at which voltage 0.3 ampere is drawn; and the tube T0 is provided with a cathode heater having a nominal rating of 25 volts, at which voltage 0.3 ampere is drawn. The cathode heaters oi the tubes TI, T2, T3 and T4 are connected inseries circuit with a 4 ohm resistor, RI and the winding of the alarm relay RI 55 across a 48 volt exchange battery, the positive pole of which is grounded. More particularly, thecircuit mentioned extends from ground 'by way, of the contacts I I9 of 2; nor mallyoperated key K2, the cathode heater of the tube T2, a conductor CII3, the cathode heater of the tube T3,.a conductor C I I4, the cathode heater of thetube T], a conductor, CI'I5, the resistor R} a conductor C116, the. cathode heater of the tube T l, and the winding, of the alarm relay R I 55 to the. negative ,pole of the 48 volt exchange battery. Accordingly,g,the, series circuit above traced is normally completed, whereby the cathode heaters heat the associated cathodes of the tubesmentionedrendering themelectron emissive and effecting operating of the alarm relay RI55 Q1 course, itwill be understood that in theevent ny ne .ofthe t be mentione i 'rem ed from its socket or develops a burned out or broken cathode heater, the above traced series circuit is interrupted, causing the alarm relay RI 55 to restore in order to complete, at the contacts I56, a circuit including, the contacts I I8 of the key K2 fpr illuminating a cathode heaters alarm lamp L I5I; thereby to indicate thefailure mentioned. More; over, the potential drop along theabove-traced series cathode heatercircuit is employed atyari ous pointsfor grid biasing purposes-explained more fully hereinafter; and, in passing, it is noted that a negative potential of 6.3 voltsappearsupon the conductor CII3, a negative potential of 18.9 yolts appears upon; the conductor CH5, and, a negative potential of 20.1 volts appears upon the conductor CII6.

The tube TI is employed as a two-way amplifier, being utilized as an outgoing amplifier and modulator in the transmission of signals to the associated repeater circuit. The screen grid of the tube TI is supplied with positive potential from a 130 volt source of plate current, the negative pole of which is grounded, through an associated 50,000 ohm resistor 129 and is by-passed to the cathode oftlie tube T] by a 0.1 mi. condenser I30 connected therebetween; and the suppressor grid of the tube TI, employed as a modulation grid, is connected either to ground potential or to the 50- 60 cy'c1e oncuetor CH2, depending upon the operated or" restored condition of the modulation cutoff relay RIZU and the receiving switch relay RI40 as explained more fully hereinafter.

For the reception of signals, the primary windof the receiving transformer TRI is connected 'to'th'e receiving" e'o'nductor CI05 and to the receiv'ing conductor C106 through the two illustrated 011 Inf. condensers; the secondary winding o thereceiyingtransformer TRI is normally connected by way of the contacts I4 I of the normally operated receiving switch relay RIM] to the control grid of the tube TI in order to control the potentialapplied thereto 'anegative potential of 20.1 volts being normallyiapplied to the control grid of the tube TI from the conductor CIIS by way of a 0115 megohm resistor R2, the secondary winding of the receiving transformer TRI and the contacts I 4| of the receiving switch relay Row. The cathode of the tube'TI is normally connected by Way of the contacts I43 of the normally operated receiving switch relay R140 to the conductor, ,CII5 having negative 18.9 volt potential applied thereto. Accordingly, the control grid of the tube'TI hasa negative biasing potential of luz volts applied thereto with respect to the cathode by virtue of the 1.2 volt drop across the resistor RI,

With the receiving switch, relay RI 4!! in its normal operated position, the suppressor grid of the-tube TI is connected by way of the con tacts I46 thereof to ground potential, as previously noted; and the secondary'winding of the sending and coupling transformerTR2 is connected, at the contacts ,I421and I44 of the operated receivingswitch relay RI 40; across the leads CI 25; and C126; The primary winding of the monitoringjtransformer TR3 is connected by way of the two 5000 ohm resistors I23 and I24 across the leadsflCI25 and CI26; while the secondary winding of the monitoring transformer TR3 is connected 'directly'to the monitoring conductor CI03 and the monitoring +conduct'or CIM extending to the associated repeater circuit. Thus, audible signals amplified by the tube TI and im-- pressed through the seriding and coupling transformer TR2 across the leads-CH5 and CIZS are supplied through the monitoring transformer TR3 to theimonitoring and'monitor'ing conductors CI03 and CHM extending to the associ ated repeater circuit, for a purpose more fully explained hereinafter; Further, the plate of the tubeTI is connected by way. of the associated 6500 ohm plate impedance I28 to the positive terminal of the 130 volt source ofplate current supply, and is also connected'by way of a 0.04 mi. coupling condenser I21 to one terminal of the primary winding -,or the sending and coupling transformer TR2, the other terminal of the primary winding mentioned being grounded.

An inductancelcoil'L3 is connected across the leads CI25 and ,CI26 iniseries with a 0.04 mi. tuning condenser I3I, which condenser is shunted by the indicated trimmer condenser of such capacity as to render the couple series-resonant at 1 000 cycles. ,Finally, th junction between the inductancejcoil L3 and the tuning condenser I3I iscOnnect'ed to the right-hand plate of the tube T2, wherebya relatively high potential is placed onjthe right-hand plate of the tube T2 by the voltage drop'across the inductance coil L3 when a .1000 cycle. current is beingreceived and amplified by the tube'TI. In order to enable the system torespond to impulses of 1000 cycles plus or minus an amount equal to about 50-60 cycles, so as to allow for the reception of 50-60 cycle modulated, 1000 cycle pulses, and to out off sharply when frequencies higher or lower than the allowable band are received, an arrangement is provided including inductance coils LI and L2 and 0.04 mi. tuning condensers I32 and I33. The inductance coils LI and L2 and the condensers I32 and I33;are arranged to'form a double-peak tuning circuit which peaks at;900 cycles and at 1100 cycles. The 900 cycle and 1100 cycle peaks are produced in addition to the 1000 cycle peak thatis produced by the inductance coil L3 and the associated tunin condenser I32. More particularly, the inductance coil L2 is arranged in series with the tuning condenser I33, which condenser is shunted by the indicated trimmer condenser of such capacity as to render the couple series-resonant at 1100 cycles, provided substantially no current is permitted to flow through an associated 0.01 mi. shunt condenser I18; which series-resonant couple, including the inductance coil L2 and the tuning condenser I33, is connected directly across the leads CI25 and CI26. Finally, the junction between the inductance coil L2 and the tuning condenser I33 is connected to the lefthand plate of the tube T2, More particularly, the inductance coil LI is arranged in parallel with the tuning condenser I32, which tuning condenser is shunted by the indicated trimmer condenser of such capacity as to render the couple parallel or shunt-resonant at 1100 cycles. In turn, the shunt-resonant couple, including the inductance coil LI and the tuning condenser I32, is connected between the left-hand plate of the tube T2 and one terminal of the shunt condenser I18; the other terminal of the shunt condenser I18 being connected directly to the lead CI25. Current of 1100 cycles is-substantially prevented from flowing through the shunt condenser I18 by the shunt-resonant effect of the inductance coil LI and the associated tuning condenser I32, as this couple is shunt-resonant at 1100 cycles. When current is received at some other frequency, however, the shunt-resonant effect is not present in the couple, including the inductance LI and the associated tuning condenser I32, whereby current is permitted to pass through the shunt condenser I18 by way of the inductance coil LI and the associated tuning condenser I32. The capacity of the shunt condenser I18 is so chosen that it, connected in multiple with the tuning condenser I33 through the system composed of the inductance coil LI and its associated tuning I condenser I32, causes the inductance coil L2'to have a resonant peak at 900 cycles in addition to the previously described peak at 1100 cycles. The manner in which the triple-peak tuning arrangement operates to control the tube T3 by way of the tube T2 is pointed out in detail hereinafter.

More particularly, the cathode of the tube T3 is connected directly to ground potential while the control grid of the tube T3 is connected directly to the right-hand cathode of the tube T2, the left-hand cathode of the tube T2 being connected directly to the conductor CH3. Also, the control grid of the tube T3 is connected to the right-hand terminal of a 0.3 megohm resistor R4, the left-hand terminal of which is connected directly to the lead CI26. Also, the left-hand terminal of the resistor R4 is connected to the right-hand terminal of a 0.15 megohm resistor R3, the left-hand terminal of the resistor R3 being connected to the right-hand terminal of the hand cathode of the tube T2.

resistor R2 previously mentioned, in other words, the resistors R2, R3 and R4 are connected in series circuit relation between the conductor CI16 and the control grid of the tube T3. Since the resistors R2 and R3 each comprise 0.15 megohm and the resistor R4 comprises 0.3 megohm, the center tap of this composite resistor is connected to the lead C I26, as previously explained. Further, the resistors R2 and R3 are by-passed by an associated 0.1 mzf. storage condenser I13, and the resistor R4 is by-passed by an associated 0.1 mf.;storage condenser I14. Accordingly, a negativebias of 20.1 volts is normally provided the control grid of the tube T3 by virtue of the connection through the resistors R4, R3 and R2 to the conductor CH6 having the potential mentioned impressed thereon; the conductor CI16 being included in the previously traced cathode heaters circuit. By virtue of the connection of the lead Cl26 to the center tap of the grid resistor, including the sections R2, R3 and R4, associated with the tube T3 and the connection of the left-hand terminal of the resistor R2 to the conductor CH6, a negative potential of 20.1'V01ts is normally maintained on the righthand plate and the left-hand plate of the tube T2 through the inductance coils L3 and L2, respectively. The right-hand cathode of the tube T2 is also supplied with a biasing potential of negative 20.1; volts by virtue of the above described connection including the resistors R4, R3 and R2 to the conductor CI16, whereby there is normally no potential difference .between the right-hand cathode and the right-hand plate of the tube T2. 011 the other hand,-the left-hand cathode of the tube T2 is connected directly to the conductor C I 13 having negative 6.3 Volts appearing thereon, whereby there is normally a negative 13.8 volts potential difference between the left-hand plate and the left-hand cathode of the tube T2. Specifically, the left-hand plate is biased with respect to the left-hand cathode of the tube T2 by the rather substantial adverse potential of 13.8 volts, whereby no current passes through the left-hand portion of the tube T2 until this rather substantial adverse bias of 13.8 volts is overcome.

During the reception of a signal, the building up of 1000 cycle potential across the inductance coil L3 causes current to fiow through the righthand plate and the right-hand cathode of the tube T2 in series with the resistor R4. Thus, half-wave rectified current passes through the resistor R4 and, together with the associated smoothing condenser I14, renders the right-hand terminal thereof more positive than the left-hand terminal thereof, thereby oiTsetting-to this extent the negative bias normally applied to the. control grid of the tube T3. On the other hand, when the signal contains a sufiicient component; of 900 cycle or 1100 cycle potential to cause a sufficient potential to be built up across the in ductance coil L2 to overcome the 13.8 volt bias normally present between the left-hand plate and I the left-hand cathode of the tube T2, current flows between the left-hand plate and the left Thus, half-wave rectified current passes through the resistors RI, R2 and R3 causing the midpoint of the composite resistor, including the resistors R2, R3 and R4, and specifically the junction between the resistors R3 and R4, to become more negative than the right-hand terminal of the resistor R4- by virtue of the potential drop through the resistors R2 and'R3." Therefore,- when a signal other than a wanted Si nal isreceived, and which c argers falls in"'the neighborhood treat cycles int- 1100 cycles, the current ifiowing' through the induct- 'ance coil L2 and "obtained through theies'istors R2 and R3,t g'ether with the associated S'rnoo'thing condenser H3, renders' 'the-control grid of the tube T3"m'ore" negative than it no'rrnally is, thereby preventing a response of the tube T3 to the signal, notwithstanding the fa'ctthat' s'ufiici'ent current may be flowing through the resistor R4 otherwise to securea' response of 'thetube' T3.

The'plate, the suppressor "grid and the screen g'rid'of the tube T3 are connected together to produce 'a triode of the desired characteristics and'are; in turn, connected both to one terminal of the associated 3300 ohm plate impedance I34 and to one terminal of the 3300 ohm winding of the impulse-in relay Ri'BiI. The other'fterminal of the'plate impedance I34 is normally connected, byway of the contacts I52 of the normally restored pulse cut-in relay RI50, to thepositive terminal of the 130 volt plate current supply; and the other terminal of the impulse-in 'r'elay R150 is normally connected, by way of the contacts I54 of the normally restored pulse cut-in relay RI50 and the 0.1 mf. coupling condenser I35, 'to the control grid of the tube T4, the junction between the contacts I54 and the coupling condenser I35 being connected by way of the 0.006 mfJby-pass condenser I31 to ground potential. The previously mentioned-grid bias of negative 20.1 volts :applied to the control grid of the tube T3 is sufli- -'cient to block the flow of current from the 130 volt source of plate current through the plate impedance I 34 under normal conditions. 'How- :ever, when a signal is received which causes the potential on the control grid of thetube T3 to be brought down to a negative value of about 16 volts, current starts to flow through the tube since the screen grid and the suppressor grid are connected to the plate of the tube T3 to securefa triode action of the desired characteristicsas previously noted. As the potential of the control grid of the tube T3 is rendered still less-negative the current traversing the plate impedance 134 is increased. In passing, it is noted that thepulse cut-in relay R150 may be operated over the associated pulse cut-in conductor C'III, thereby to interrupt, at the contacts I52, the previously traced circuit, including the plate impedance I34, between the 130 volt source of'plate current and the plate ofthe tube T3; and to complete, at the contacts I5I, an obvious substituted circuit, including the winding of the impulse-in relay R160, between the 130 volt source of 'plate-current and the plate of the tube T3. Also, upon operating, the pulse cut-in relay R150 interrupts, ;at the contacts I54, the previously traced'connection, including the winding of the impulse-in relay RISE, between the plate of the tube T3a'ndthe coupling condenser I35; and completes; at' the contacts I53, a substitute circuit, including the plate impedance I34,'between theplate of the tube T3 and the coupling condenser I35.

When the pulse cut-in relay R'I50"occupies its operated position and a signal is'received,*the potential of the control gridbf' the tube T3is rendered less negative, as previously -'e2 plained, whereby the plate current of the tube T3traverses the winding of the impulse-in relay R160; and when this platecurrent'reaches a value of about fifirnilliampere the impulseinrelay R150 is sufficiently energized to operate; about 9 niilliam'pere being preferable for maintainingbperated"the impulse-in relay "R I 30 duringfthe' reception-" ot dial pulses. "upon cperating, the'iuipfllswin'rclay R1 50 icom'ple'tes, at the contacts I 61 I an tenets path for' applying ground pbtntial to "thedialin conductor CHO extending to the associated repeater circuit. 'Sincethef 'inductance coil L3 5 andthe associatedtuning ctindefisei I3'I comprise a resonant couple having the maximum response at 1000 cyclesQbutnot sharply peaked, the doublepeak inductance coil I192 associated with the left-hand plate and lft hand cathode of the tube T2"offers an opposing efiect to afford definite frequencycutoff points'above and bl'o'w'the desired1'000 cycle res'onant point while there is ara'th'er'fiat frequency response over the selected band. "This band is 'about"100=120 cycles widein order to provide for'the-r'ecepti'on of 50430 cycle modulated, 1000 cycle signal-impulses." By arrangingthat the response curves'of the inductance coils L3 and 'LZintei'm'ingIe, theindu'ctance coil L2 peaks at900 cycles and=at 11'00 cycl'es'but still builds up an 'app'reciable voltage at 1000'c'y'cles. The'voltage built up across the double-peak inductance coil LZ'at the 1000 cyclesignal frequency and over the "upper portion of the signalstrength range is sufiicient'to' reduce considerably the positive grid-swing tendency of the control grid of the tubeT3, thereby'tending to secure a uniform response over a wide range of input voltage. The weak "signals, however, are not handicapped by the opposing effect of the inductance coilLZ as the adverse" bias is'not then overcome.

' However, in order-positively to'p'revent damage to'the various tubes employed in the tone 'unit100, the control grid of the tube T3 is arranged to cause rectification Whenstrong 1000 cycle signals are received. More-particularly, when strong 1000 cycle signals are 'receivedf-the current traversing the right-hand portion of the tubeT2'becomes sufiiciently large to cause thepotentia'l drop across the resistor R4 to become'sufiiciently large so that the control gridof the tube T3 isrendered positive with respect to'the associated cathode thereof, whereby rectification at the control grid of the tube T3 takes" place. This icontrol'grid rectification in'the tube T3 "causes a potential drop across-the resistors B L-R3 and R2 that is proportional to'thestrength'of the incoming 1000 cycle signal. The voltage drop across'the resistor R2 adds to the voltage drop ac'ross the resistor RI; thereby increasing the negativeybias applied to :thecontrol gridof the tube TI. This increased negative bias "applied to thecontrol grid of the tube TI on strong 1000 cycle signals reduces the output thereof "and the 'consequentresponse 55 thereof "to thestrong' 1000 'cyclesignals; whereby the'c'urrent through the right-hand section of the tube T2'is proportionallyreduced, rendering less positive thebi'as applied to the control-"grid of the tube T3, Thus, this arrangementpreventsover- 60 loading of the tubes TI, T2 and T3 .in-response to therecep'tion of strong 1000 cycle signals.

"The cathode and thesuppres'sor gridof the tube T4 are connected directly-to ground potential while the control gridof the tube T4is provided with anorm'al negative -biasing-potential (Jr-20.1

volts through 'a'0.1megohm resistor ISBconnected thereto and to the conductor CI'IG'haVing the negative potential mentioned thereon. "Moreparticularly, the'control grid of thetube T4 is connected'to the junction between 'the resistor I35 andthe coupling condenser I35. 'The' plate-of the tube'Tl'is'conne'cted directly 'by way of the 1000 ohm plate impedance I39 to the 130 voltsourceof platesupply;- and the screen grid ofith'e-tube T4 is connected dirctly to' the 1'30 *vo1tfplate1supply.

9 At this biasing potential substantial plate current flows through the plate circuitof the tube T4 and the plate impedance I39 from the 130 volt source of plate current; the tube T4 being connected to provide the usual form of pentode operation. The coupling condenser I35 serves to couple the plate of the tube T3 to the control grid of the tube T4 in order to transmit the 50-60 cycle signal impulses to the tube T4. After amplification by the tube T4 the signal impulses pass to the 600 ohm winding of the signal-in relay RI'I0 by way of a 0.3 mi. coupling condenser I38. The capacity of the coupling condenser I38 and the inductance of the winding of the signal-in relay RI'I0 are so chosen that this circuit containing them constitutes a series-resonant circuit at 50-60 cycles, whereby this couple responds vigorously to 50-60 cycle current but does not respondwhen the 1000 cycle dial impulses, following eachother at about ten to fourteen impulses per second, are received. When the series-resonant circuit, including the coupling condenser I38 and the winding of the signal-in relay RI I0, responds vigorously to the 50-60 cycle current mentioned, the signal in relay RII operates in order to complete, at the contacts III, an obvious path for applying ground potential to the signal-in conductor'CIUS extending to the associated repeater circuit. The by-pass condenser I3! is arranged to by-pass to ground high frequency currents in the plate circuit of the tube T3 in order to prevent these frequency currents from being impressed upon the control grid of the tube T4.

Considering now the operation of the tone unit I00 in conjunction with the associated repeater circuit, when the tone unit I00 is in its normal position the modulation cutoff relay RI20 is normally restored and the receiving switch relay RI III is normally operated, the receiving switch relay RI40 being operated in a circuit including the grounded receiving switch conductor CIO'I extending to the associatedrepeater circuit and the contacts I I6 of the normally restored key KI Also, the pulse cut-in relay RI50 is in its normally restored condition whereby the impulse-in relav RIBU is disconnected from and the plate impedance I34 is connected into the plate circuit of the tube T3. This is the condition of the tone unit I 00 when it receives a 50-60 cycle modulated 1000 cycle seizing pulse from the associated re eater circuit when the associated toll line is taken foruse in the distant exchange. This pulse comes into'the tone unit I00 through the receiving and receiving conductors CI05 and CI06 and passes through the receiving transformer TRI to the grid-cathode circuit of the two-Way amplifier tube TI. The signal is amplified by the tube TI and passes through the coupling condenser I2'I through the primary winding of the sending and coupling transformer TRZ. From the secondary winding of the sending and coupling transformer TRZ the signal passes through the contacts I 42. and I44 ,of the operated receiving switch relay RIM and is impressed between the signal-in relay 'RIIII, causing the latter relay to respond to the flow of alternating current therethrough. The signal-in relay RI'IO responds since thewindingf thereof, together with the coupling'condenser I38, constitutes a couple resonant at the 50-60 cycles of the modulating signal, as previously noted. Upon responding, the signal-in relay RI'I0 operates to complete, at the contacts "I, the previouslymentioned path for applying ground potential to the signal-in conductor CI09 extendingto the associated repeater circuit; which ground potential is maintained upon thesignal-in conductor CI09 as long as the signal-in' relay RIIUremains operated, which condition pr'evails for the duration of the reception of "the 50-60 cycle modulated signal. At the conclusion ofthe 50-60 cycle modulated seizing pulse the signal-in. relay RI'I0 restores in order to interrupt,'at the contacts I1 I, the application of grou'nd potential tothe signal-in conductor CI09 extending to the associated repeater circuit. I

When the associated repeater circuit responds to the seizing pulse it applies ground potential to the pulse cut-in conductor CI I I, whereby the pulse cut-in relay'RI50in the tone unit I00 is operated. Upon operating, the pulse cut-in relay RI50 substitutes theimpulse-in relay RISE for the plate impedance I34in the plate of circuit of the impulseamplifier tube T3, as prethe circuits in the tone unit I00, in the manner previously explained, whereby the current in the plate circuit of the impulseamplifier tube T3 effects operation of "the impulse-in relay RIGU. More particularly, the impulse-in relay RIG!) is operated momentarilylupon the receipt of each ofthese dial pulses and'restores between the reception of adjacent dial pulses. v Each time the impulse-in relay Rlfifloperates and then restores it completes and theninterrupts, at the contacts I6I, an obvious p'athifor applying ground potential to the dial-in conductor CI I0 extending to the associated repeater circuit. The dial pulses in the plate circuit of thetube T3 are received at about ten to fourteen per second and pass through the plate impedance I3=4,the contacts I53 and the coupling condenser I35 into the control gridecathode circuitof the signal amplifier tube T4. These dial pulses are amplified by the signal amplifier tube T4 and pass into the -60 cycle theleads CI25 and CI-26. This signal then passes resonant circuit including the coupling condenser I38 and the winding of the signal-in relayRI'Ili. However,'the resonant circuit mentioned does not respond appreciably to the amplified dial pulses and, accordingly, the signalin relay RIIIJ does not operate.

J I34 in the plate circuit of the impulse amplifier tube'rs; thereby positively preventing further response by the impulse-in relay Rl'60 while permitting the signal-in-relay RI'IIl to respond to an-y'fu'rther received current at 1000 cycle carrier freque'ncywhich is modulated'at the predetermined 50-60 cycle rate, as previously explained.

When the tone-cir "it-J00.is to be used to i 's fiii eii w ill es? 1 and repeater, circuit' ove'r th atedtoll, l ne, the receiving switchIrelay Rl 3- 31181110611 contacts I49 and, I91; the revious ly mentioned ec i n'b wf i, l. i. =6 a ic'gri p w way amplifier tubeTl ahu tne lilflcycle c'onti ol conductor C'I I3, Also, the receiving switch relay RM!) interrupt' s jatftheico' ntacts"[46, the'p'reviously traced pathff rpr applyingfl ground potential tothe suppressor orlmodulation grid of the two-way amplifier tube'I lg andcornpletes, at the contacts I45, thepreviously; mentioned connection, including the contacts-1 21', for connecting the suppressoror:modulationjgrid of the twoway amplifier t'ube 'Il tPlQhQ 505 60 cycle control conductor Cl [2, Also,-' thereceiving switch relay Rl lfi interrupts; at thQcpnt t'sf [42 and I44, the previously traced conljlcction'gbetween the secondary winding of" the; sending and coupling transformer TRZ. nq'tii jea c I 25 and 012s; and completes, at the' contact's F41 and M3, an alternative connectioln' hetween thefsecondary winding of the sendingandj coupling transformer TRZ and the sending aridsending conductors CHM and Ql'021extendlrigj to the associated repeater circuit. At thisjpointjitis'noted that a 2000 ohm adiu le' re'sisjtor llzris connected between the' sen'din andfthe"siending conductors Cl 0 andlg extending totheassociated repeater. circuitjiniordei to control the output level of the signal iro n the tone unit me. At this time' ajroqo cyclfe'variation is produced in theplate currenlfof the two-way amplifier tube 'lf l" and pas through the plate impedance l2? the e; tim e the suppressor or modulationgridof the wo' wayamplifier tube Ti connected to the 5Q 6Ocycle' control conductor CH2 causes, on each negative halfflcycle of the potential on thecontrolconductor Cl [2, the current flowin theflpla'te circuit ofthe two-way amplifier tube Tl to c'ea se'; altogether; while on P Fe} l i he each positivehalf cycle of the potential. on the control conductor 011g, theiplatecurrent in. the two-way amplifier t'ube fIllis permitted to flow unimpeded. In this manner, fiity-sixty uniformly separated pulses of. 1000" cycle current are caused tofidyv' throu'gh .theplate circuit of the two-way amplifiertube Tl through the plate impedance [28; which pul sesj are permitted to pass through the'coupling condenser l2! tothe primary winding or the sending and coupling transformer TRZ S inc'elthe secondary" winding of the sending= ancl; coupling transformer TR2 is connected', hy way ot theabovetraced path, to the sending and sending conductors- Citl and Ql llz extendingl to; the associated repeater circuit, the 0- -6fl cycle modulated 1000 cycle current is transnritted oven the sending and sending conductors-Cl! andiclnz to the associated repeater circuit andthence over the associated' toll line. Atti e conclusion of the transmission of .the' seizing pul the receiving switch relayita's eoperate' y the'associated repeater ,circuit'inlorderi to retiirnthe tone unit I00 to the normally exis'ting receiving condition previously expl'ainedI v In order to preparefforlthe transmission ofan unmodulated 1000 cycle signal froni the tone unit we to the associated repeater-circu t, b'attery potential is applied in the associated repeater cir cult to thernodul ationv cutoff, conductor C1118, thereby to eficct operation of the Inodiilationcutofi relayRiZll in thegtonepnit I UD'I Upon oper ating, the: modulation cutdflf"1'ela y Rl2flinter rupts, at the contacts; 121 a further pointin the previously traced path: for connect rig the sup pressor grid of the twoe'wafy air puner ube TI to the 50-60 cycle contrio'l'. conducton'ill 1 2; and pre pares, at thev contact- 122; alipathtrajcedihereinafter for connecting directly t ground'potential the suppressor grid of thetwo-way amplifier tube T I; Subsequently when the associated repeater circuit is in readinesstbfreceive"the'unniodulated i) cycle signal rromtn tone unit (Q0, ground potential is removed; fr"orn the receiving switch conductor Clfl'l thereby t the receiving switch relay RMil' inthe QUto restore. Upon restorin the receivingis relay" R140 completes, at the contacts l l'lf an f,thep're'- viously traced path foif connecting; the control ri e w -Wei me fi l be fiot e cyclei control conductor Cl;lf l; and completes, at the'cdntacts M5; the previously mentionedpath, c ud n ,t llconta ts 1 or e eq to rowi roie ie h is' finfs' r f th amp fie ftu ei i-q s'a on r h e ei in w ch r ay-R14 i p at the c nta 141 nd fls t e q sll l ra v nneq im ew re the e oiida t i wind n o the sendi 1g and couplingtransforpier IE2: and the e di and: end n dli brsp 0t d X end lne'iQ s. t fr p e 'c b' i s m unmqeal, ed l o yt -eurr ing i h P at ri u t o th twewa amplifier #11108 1'. P s s ihl lQ eh.tlie lnP con I21 andthe primary windi'n of thesending and c m i s v n i mer IR. wher the S da y Wind ng of; the sending and coupling transformer T32 DI'OjQC tS anI unnggdlulated i000 cycle signal over the send ingfl sending .con ductors cm and ciq2 t6 th associated re peater circuit. It willbe'understood that the unmodulated 1000 cycle-signal transmitted from the tone unit I00 to the lassociatedirepeater circuit is connected'a'nd discpfineetd iin the a'sso-" ci'at'ed repeater circuit to the; t n. line; associated There after: when the apparatus as-' sociated withthe repeatercircuitiin the distant exchange extends the connec 1 an end'of dialing signal is returned over thetolllline. totherepater circuit associatedwitli' tnewne umt I i=3" which 'end-v of; dialing; signal is in the form-of a 50-60. cycle modulated 1000: cyclesignal; which signal received by the tone unit Ivefiects opera tion oi the signal-inrelay R110, all in the manner previously explained. Also, it is noted that this 50-60- cycle modulated 1000; cyc1esignal is first received by thereceiving transformer 'I-Rt and amplified by the two-way amplifier tube TI and then applied by the sending and coupling transformer TR2 between the leads CI25 and CI26; Since the primary winding: of the monitoring transformer TR3 is connectedby way of there'- sistors I24 and I23 across the leads CH and CI 26; this audible signal passing through. theprimary winding oi themonitoring transformer TR3- causes a corresponding; audible signal to be-trans mitted' from the secondary winding: of the moni toring transformer TR3 over the monitoring. and monitoring conductors CI'03: and CI 04 extending to the associated repeater circuit. Accordingly, inthe event'the monitoringv and monitoring: conductors C I 03- and: CI 04 extending to the associated repeater circuit'are' at this time connected to an associated; operator position, the operator thereat may hear through her headset theaudible signal; mentioned; This ar-- rangement gives the operator, at the operator-pe sition= associated with the repeatercircuit that is in turn associatedrwith the'one unit I00, monitor-- ing supervision of the connection as it is extended over the: associated" toll line; without interfering in any way with the-subsequent reception ofsuper visorysignals over the. tollline associated with the repeater circuit with which the-tone unit I00; is

associated? For example, following the reception 1 of the endoi dialing signal abovetmentioned; the repeater circuitassociated withftlie toll line in the distant exchange may transmit a; supervisorysig-- nal indicating a'l busy'c'ondition in thedi'starIt eX- change, which supervisory or busy signalis theform-of a-60 cycIe' modulated 1000 cyclesignali This busy signal,.returned over the tollli'neassociated with the repeater circuitwith which the tone unit I00 is asso'ciated;is received by'the tone: unit in order to'cause operation of the sign'al in relay R H 0" therein, in: the manner'previously eX-- plained', as wellas to cause this audible signal to be returned via the monitoring transformer PR3 to the operator position associated 'with the repeater circuit with. which the tone unit I00 is associated; alliwithout interference.

In the event it is desirable during repair or test of the tone unit. I 00 to open the cathode heaters circuit, the keyKzmaybe restored 'for' this purpose; When thekeyKZ is' thus restored there is interrupted; at the-contacts I-I8 thereof; afurther point in the previously traced circuitl for illuminating" the cathodeheaters alarm lamp: LI51',thereby to preventlillumination thereof incident to the restoration of the alarm relay RI55. Subsequently, the'key K2 is returned to its normal operated position in order to; close/the cathode heaters circuit and to bring about reoperation of the alarm relay RI55. V

In the event that it isdesired to test the transmission level of the tone :unit I00, the key may be actuated and a test plug associatedwith the testequipment may be inserted into the j'ack JI connectedacross the sending and sending [-iconductorsCIUI and CI02. Whenthe hey Kt is thus operated there is interrupted, at the contacts H6 thereof, the previously traced circuit for maintaining operated the receiving switch relay RI40 in order to cause the latter relay to restore; and there is completed, at the contacts R140 reoperates;

I11 thereof; a series circuit ,for operating; the modulation cutoff relay R and for illuminatingthe cathode heaters alarm" lampLI 51. When the modulation cutoff relay BI 20 is thus operated and the receiving switch relay RI40 is thus restored; thetone unit I00 is placed in-its transmitting; position, whereby'anunmodulated 1000 cycle signal is transmitted over the sending, and sending: conductors CI0I' and CI 02'; which signal effects operationof theitest equipment associated with the'jaclrJI. At this time-theresistor I12 should be so adjusted that the output level of the unmodulated 1000 cycle signal received by the test equipment associated with the jaclr J I is 10 db. from'0.006 watt. At this time the test equipment may be removed from the as-- sociated jack J In and the key KI returned to its normal position, whereby the modulation cutoff. relay R120 restores and the receiving switch relay At this time the tone unit I00- is returned to its normal'existing receiving condi tion'.

Finally, it is noted that the connection betweenthe control grid of the two-way amplifier tube- TI and the 1000 cycle control conductor CH3:

includes=the contacts I?" and I49 of the-receiving switch relay R in: series-circuit relationship; which: connection when interrupted; at thecontacts I41 and. I09; causes one of the contact springs of the contacts I41 to be' grounded at the contacts I48:. This arrangement reduces the intercontact capacity which mightsotherwise be; sufficient to' cause; a? slightlocal- 1000 cycle-voltage totb-e impressedupon.thercontrol grid of the-two-- way amplifier tube TI evenwith the; receiving switch relayRI-kilin-its operated position This tendency is,, of; course, positively prevented by the grounded: contacts I48 disposed intermediate" the contacts I41: and. I49 of' the receiving; switch relay RIM).

v In View oft'he' foregoing, it is apparent that there has beenprovided an improved tone unitcapable of receiving unmodulated' 1000 'cycle dial pulses, as well as 50-60 cycle modulated 1000' cycle supervisory signals; and capable of transmitting aniunmodulated1000-cycle signal for dialing purposes, as well as 50-60 cycle modulated 1000 1 cycle 1 supervisory signals; Also, the tone unit comprises an improved arrangement for preventing damage as a-- consequence otover-loading of the varioustubes incorporated therein in response to the re-- ception of strong: 1000 cycle signals; as well asanimproved arrangement for permitting monitoring over a toll line: associatedv with a repeater circuit with which the tone unit'is associated;

while the tone unit isconditioned to receive supervisory signals from the associated repeater circuitover the-associated tollline; without'interference;

Whileztherei has been described: what is'at presentconsidered to" be the preferred embodiment ofith'e'invention; it"will'hezunderstood thativarious modifications may be'made thereinrand 'it is intended to cover: in the appended claims all: such P modifications as fallwithin the true; spirit and network, means including an amplifier for cou- V pling said receiving channel to said circuit network, a monitoring channel, means including an impedance element for coupling said circuit network to said monitoring channel, a rectifier operatively connected to said circuit network and. operative to produce during said given time interval direct current impulses at said predetermined frequency and to produce direct current impulses at said given rate, a local circuit, first means controlled in response to said direct current impulses at said predetermined frequency for repeating during said given time interval a signal pulse into said local circuit, and second means controlled in response to said direct current impulses at said given rate for repeating dial impulses at said given rate into said local circuit.

2; In a signaling system, a receiving channel adapted to receive during a given time interval a predetermined frequency modulated audio frequency current, a circuit network, means including an amplifier for coupling said receiving chan nel to said circuit network, a monitoring channel, means including an impedance element for coupling said circuit network to said monitoring channel, means including a rectifier operatively connected to said circuit network for producing during said given time interval direct current impulses at said predetermined frequency, a local circuit, and means controlled in response to said direct current impulses at said predetermined frequency for repeating during said given time interval a signal pulse into said local circuit.

3. In a signaling system, a receiving channel adapted to receive dial pulses at a given rate of audio frequency current, a circuit network, means including an amplifier for coupling said receiving channel to said circuit network, a monitoring channel, means including an impedance element for coupling said circuit network to said monitoring channel, means including a rectifier operatively connected to said circuit network for producing direct current impulses at said given rate, a local circuit, and means controlled in response to said direct current impulses at said given rate for repeating dial impulses at said given rate into said local circuit.

4. In a signaling system, a receiving channel adapted to receive audio frequency signals, a circuit network, means including an amplifier for coupling said receiving channel to said circuit network, a monitoring channel, means including an impedance element for coupling said circuit network to said monitoring channel, an electron discharge device including input and output electrodes, an output circuit coupled to said output electrodes, a first tuned circuit coupled to said circuit network and having a frequency response characteristic that. peaks at a predetermined audio frequency, a second tuned circuit coupled to said circuit network and having a frequency response characteristic that peaks at two audio frequencies respectively disposed above and below said predetermined audio frequency, means controlled in accordance with the signal voltage across said first tuned circuit for impressing a first control voltage between said input electrodes, and means controlled in accordance with the signal voltage across said second tuned circuit for impressing a second control voltage between said input electrodes, said first and second control voltages being in opposition to each other so that the current in said output circuit is controlled in accordance with the difference in magnitudes of the signal voltages across said first and second tuned circuits.

5. In a signaling system, a receiving channel adapted to receive during a given time interval a predetermined frequency modulated audio frequency current, a circuit network, means including an amplifier for coupling said receiving channel to said circuit network, a monitoring channel, means including an impedance element for coupling said circuit network to said monitoring channel, a conductor, means for impressing a biasing potential upon said conductor, means including a first rectifier operatively connected to said circuit network for impressing during said given time interval impulses of signal potentials at said predetermined frequency on said conductor, said signal potentials being in opposition to said biasing potential on said conductor, means including a second rectifier operatively connected to said conductor for drawing current therefrom when the signal potential overcomes the biasing potential on said conductor, means controlled in response to the drawing of current from said conductor by said second rectifier for controlling said amplifier to increase the coupling impedance between said receiving channel and said circuit network, a local circuit, and means controlled in response to said impulses of signal potentials at said predetermined frequency on said conductor for repeating during said given time interval a signal pulse into said local circuit.

6. In a signaling system, a receiving channel adapted to receive dial pulses at a given rate of audio frequency current, a. circuit network, means including an amplifier for coupling said receiving channel to said circuit network, a monitoring channel, means including an impedance element for coupling said circuit network to said monitoring channel, a conductor, means for impressing a' biasing potential upon said conductor, means including a first rectifier operatively connected to said circuit network for impressing impulses of signal potentials at said given rate on said conductor, said signal potentials being in opposition to said biasing potential on said conductor, means including a second rectifier operatively connected to said conductor for drawing current therefrom when the signal potential overcomes the biasing potential on said conductor, means controlled in response to the drawing of current from said conductor by said second rectifier for controlling said amplifier to increase the coupling impedance between said receiving channel and said circuit network, a local circuit, and means controlled in response to said impulses of signal potentials at given rate on said conductor for repeating dial impulses at said given rate into said local circuit.

OTHO D. GRANDSTAFF.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,155,176 Wicks Apr. 18, 1939 2,159,081 Grandstaff May 23, 1939 2,331,360 'Tuckerman Oct. 12, 1943 2,363,985 Moser Nov. 28, 1944 Certificate of Correction Patent No. 2,482,478 September 20, 1949 OTHO D. GRANDSTAFF' It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows:

Column 16, line 57, before the word given insert said;

and that the said Letters Patent should be read with this correction therein that'the same rnay conform to the record of the case in the Patent Ofiice. Signed and sealed this 31st day of January, A. D. 1950.

THOMAS F. MURPHY,

Assistant Gammz'ssz'oner of Patents. 

